You're not going to have much success if there's too much movement in the floor no matter how good the adhesive is. How is the ply fixed to the joists? What's the spacing of the joists? Have you fitted noggins under the joints in the ply?
The first person I asked said the super flex adhesive would be 40 quid a bad, the second said there was a two part adhesive for 25 quid a bag, I just wanted to check that the first bloke was chrging too much, and they were talking about the same product (I am now going with the checper bloke, I just wanted to use the best adhesive.)
Oh you've over boarded with 18mm ply. I see. I assumed you'd removed the floorboards and laid 18mm ply. 18mm ply is OTT when overboarding - that should be solid as a rock.
How have you screwed it down? You would screw 12mm ply down every 200mm (8") all over the sheet. With 18mm I'd reckon you'd be ok every 250mm (10").
If you've screwed it down properly it should be very stable. If it's still moving then I would suggest that the floorboards need fixing down better.
What's the area you're doing? If it's a small area, would it be possible to remove the floorboards and just screw down the ply (adding extra noggins)? Even better would be to rip out what you currently have and put down 25mm ply (exterior grade) and fix noggins (extra supports) under the joints in the ply.
Adhesives from here: http://www.trades-direct.co.uk/modules/shop/products.asp?rangeid=104
The tiler is going to put down the ply so I will see how much movement is left after this.
It is an old house and some of the joints are not like the timber we know today.
The area is 3600 x 3600 with a couple of recesses and its hard to measure the movement but I wrekon it could be up to 10mm when I stand on my toes and drop
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